Grinding mills for reducing the size of particles of a material to a predetermined maximum particle size are known. Typically, a charge (e.g., ore and water) is added into the grinding mill at an input end of a shell or mill of the grinding mill. The mill is rotated. Typically, the mill includes shell lifter bars positioned lengthwise on the mill, to carry a portion of the charge upwardly as the mill rotates. The portion which is carried upwardly is allowed to fall onto the balance of the charge, thereby providing a tumbling effect, to grind or comminute the material to the predetermined maximum particle size or less. After the particles are reduced to the predetermined maximum particle size or less, such particles are discharged from the grinding mill at a discharge end thereof, e.g., via a centrally-mounted discharge trunnion, as the mill rotates.
Certain aspects of the prior art discharge end liners are illustrated in FIGS. 1 and 2. (As will be described, the remainder of the drawings illustrate the present invention.) A schematic illustration of a discharge end liner 20 of the prior art is provided in FIG. 1. As can be seen in FIG. 1, the typical grinding mill includes a number of pulp lifters 22 which are positioned on a discharge end wall (or mill head) 41 in a pattern around an inner opening 24 in the discharge end wall, and which define chambers 26 therebetween. (It will be understood that various features of the prior art discharge end liner have been omitted from FIG. 1 for purposes of illustration.) As is well known in the art, the discharge end wall is rotatably mounted on a discharge end trunnion, and the inner opening is located around the discharge end trunnion and coaxial therewith. Often, the head is positioned at an angle (e.g., 15° relative to a central axis (not shown in FIGS. 1 and 2) about which the shell and the discharge end wall rotate, i.e., the head may form a truncated cone. As is known, the charge typically is positioned in a lower part of the shell only up to a limited height, i.e., the region generally designated as “X” in FIG. 1. In FIG. 1, the direction of rotation is indicated by arrow “A”.
The charge moves gradually to the discharge end as the mill rotates. As is well known in the art, the mill may be autogenous or semi-autogenous, i.e., the charge may include other materials (e.g., balls) for improving the grinding action.
Also, as the mill rotates, each chamber 26 is moved between a submerged condition (in which each chamber is at least partially submerged in the charge respectively) and a raised condition (in which each chamber is raised above the charge respectively).
In the prior art, the pulp lifters 22 support one or more grate subassemblies 42 (FIG. 2) which are spaced apart from the discharge end wall, to partially define the chamber 26. Each grate subassembly 42 includes one or more grates, and typically also includes one or more blind plates. Each grate (or grate plate) has holes in it sized to allow particles of the maximum particle predetermined size or less to pass therethrough when the particular chamber is in the portion of its arc in which the charge engages the grate, i.e., when the chamber is in the region identified as “X” in FIG. 1. Accordingly, because of screening by each grate, only relatively fine particles in a slurry (as well as liquid) are allowed into each chamber 26.
For example, as can be seen in FIG. 1, slurry flows into a particular chamber when the chamber is between approximately the 8 o'clock and the 4 o'clock positions (i.e., when the chamber is in the submerged condition). Also, slurry flows out of each chamber and into the inner opening under the influence of gravity when each chamber is approximately between the 3 o'clock and the 9 o'clock positions (i.e., when each chamber is in the raised condition).
Each chamber 26 has an exit port 28 at an inner end 30 thereof, through which the slurry is discharged from the chamber. Preferably, upon discharge, the slurry is directed by an outer wall of a discharge cone 31 (FIG. 2) through the discharge trunnion (not shown), to exit the grinding mill.
The prior art pulp lifters have some disadvantages. In the prior art, significant discharge typically does not actually begin until the chamber is at approximately the 12 o'clock position, and (as indicated above) discharge must be completed by the time the chamber has reached the 9 o'clock position. In the event that the evacuation of the slurry from a chamber has not been completed (or substantially completed) when the chamber reaches the 9 o'clock position, slurry remains in the chamber, thereby effectively reducing the capacity of the grinding mill. In the prior art, failure to completely discharge (or substantially completely discharge) can result in a substantial proportion of the chamber being occupied by undischarged slurry, in turn resulting in substantially increased operating costs.
Typically, the mill is rotated at a relatively high speed, to achieve optimal throughput. For example, a typical mill with an internal diameter of about 32 feet (approximately 9.8 meters) may rotate at about 10 revolutions per minute. Although decreasing the rotation speed would facilitate discharge from each chamber, such decrease is understood to be counterproductive, as any such decrease would also decrease throughput, as is well known in the art.
One attempt to address the problem of failing to substantially evacuate the chamber is shown in FIG. 1. In FIG. 1, adjacent pulp lifters (identified in FIG. 1 with reference numerals 32, 34, 36) have different lengths, i.e., they are terminated at different distances from the inner opening 24. The disadvantage of this approach is that the flow of slurry out of one chamber may obstruct the flow of slurry out of an adjacent (lower) chamber. For example, in the position shown in FIG. 1, flow of slurry out of chamber “Y” (between pulp lifters 32 and 34), schematically represented by arrow “B”, is obstructed to an extent by slurry flowing out of chamber “Z”, represented by arrow “C”.
A cross-section of a typical pulp lifter assembly 22 of the prior art is provided in FIG. 2. In FIG. 2, the pulp lifter assembly 22 (and the chamber 26 partially defined thereby) is shown in the raised condition. The prior art pulp lifter assembly 22 includes, for example, one or more outer lifter segments 38, and one or more inner lifter segments 40, which are mounted onto a discharge end wall 41, which is attached to an outer wall 39 of a shell (or mill) 52. The grate subassembly 42 is positioned on the lifter segments 38, 40. As is well known in the art, the grate subassembly 42 typically includes one or more grate plates 44 (i.e., with appropriately-sized holes 45 therein, as described above) and a blind plate 46. (It will be understood that the holes 45 are represented in FIG. 2 much larger than is appropriate, to simplify the illustration. As is well known in the art, the holes 45 are generally relatively small.) The blind plate 46 does not have holes in it, so that it assists in directing slurry toward the inner opening, when the chamber is in the raised condition. An outer wall 48 of the central cone 31 is also shown in FIG. 2. As can be seen in FIG. 2, when the chamber 26 in the raised condition 26, the slurry therein flows under the influence of gravity in the direction generally indicated by arrow “D”, to exit the chamber 26, subsequently to be directed by the outer wall 48 (i.e., in the direction indicated by arrow “E”) to the inner opening 24, and out of the grinding mill.
In practice, the slurry exiting the chamber as indicated by arrow “D” in FIG. 2 is only a retained portion of the total volume of slurry (the “inflow volume”) which had entered the chamber while the chamber was in the submerged condition. Such retained portion typically represents a major part of the inflow volume of slurry. As is known, a backflow portion of the inflow volume backflows into the shell (i.e., into the charge) while each chamber is being raised out of the submerged condition toward approximately the 12 o'clock position. In general, backflow tends to be minimal between the 12 o'clock position and the 9 o'clock position. The retained portion is the balance of the inflow volume remaining in the chamber once the backflow volume is taken into account.